Introduction

Negative resistance models the required gain needed from the active network to sustain stable oscillations. The negative
resistance model is shown in Figure 1. The crystal is represented by a reactance term Xm and a motional resistance term Rm.
The active network is represented by the reactance term Xosc and the Resistance term -Rneg where Rneg is the negative
resistance of the inverter amplifier.

One process used as a means to easily evaluate the negative resistance characteristics and oscillation allowance of an oscillator
circuits is the method of adding a resistor to the hot terminal of the crystal unit and observing whether it can oscillate (examining
the negative resistance). The oscillator circuit capacity can be examined by changing the value of the added resistance.

Measuring Negative Resistance of the Active Network

The steps for measuring the negative resistance of the active network are as follows:
1) Compute crystal parameters Co, Cs, Ls, Rs and CL using a Network Analyzer such as 250B/C by Saunders and Associated
Inc.
2) Solder crystal on X1 and X2 pins of the IDT clock IC and mount the crystal load capacitors next to the device pins (as shown
in Figure 2).
3) Adjust the load capacitance to get the output frequency error to nearly 0ppm (account for contribution due to stray capacitance
on the PCB board).
4) Connect a series resistor Rx (as shown in Figure 2) and keep increasing its value until oscillations stop occurring on powering
up the device.
5) The negative resistance of the active network is then equal to Rx + Rs.